Light olefins, defined herein as ethylene and propylene, serve as feeds for the production of numerous chemicals. Olefins traditionally are produced by petroleum cracking. Because of the limited supply and/or the high cost of petroleum sources, the cost of producing olefins from petroleum sources has increased steadily.
Oxygenates such as alcohols, particularly methanol, dimethyl ether, and ethanol, are alternative feedstocks for the production of light olefins. Alcohols may be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for olefin production. Such conversion processes are referred to as oxygenate to olefin (OTO) conversion processes and occur in OTO reaction systems. In an OTO reaction system, an oxygenate in an oxygenate-containing feedstock contacts a molecular sieve catalyst composition under conditions effective to convert at least a portion of the oxygenate to light olefins, which are yielded from the reaction system in a reaction effluent.
One type of reaction unit useful for conducting an OTO conversion process is a fluidized bed reactor, wherein solid catalyst particles contact a fluidizing medium, which causes the solid catalyst particles to become suspended in a fluidized state during contact with the feedstock and other vapor materials. Steam and/or inert gases typically serve as fluidizing mediums.
Typically, undesirable by-products are formed in OTO reaction systems in addition to the desired light olefins. One method for reducing the production of undesirable by-products in a fluidized bed reactor involves operating in a hydrodynamic flow regime such that the superficial gas velocity obtains a velocity high enough to cause a net flow of catalyst in the reactor in the same direction as the flow of the feedstock and other vapors. That is, the feedstock and other vapors essentially carry the catalyst particles along with them. These flow regimes are known to those skilled in the art as fast-fluidized bed and riser regimes, and are preferred in reaction systems in which a more plug flow reactor type is desired.
Preferred OTO catalyst compositions, which exhibit desirable fluidization and conversion characteristics, include metalloaluminophosphate molecular sieves, e.g., silicoaluminophosphate (SAPO) molecular sieves. Activated metalloaluminophosphate molecular sieves have been found to be sensitive to moisture. In general, significant exposure of the activated molecular sieves to moisture has been found to deactivate the catalytic activity of the molecular sieves. Thus, fluidizing such catalyst compositions with steam as the fluidizing medium may decrease catalyst composition lifetime. As a result, the need exists for fluidizing catalyst compositions that include metalloaluminophosphate molecular sieves without deactivating the molecular sieves contained therein.
In addition, a portion of the undesirable by-products formed in OTO reaction systems includes C4+ olefins. These materials are, in general, less valuable than the desired light olefins, and in some circumstances they may be difficult materials for which to find a market at all. Thus, there is also a need for improved methods to manage these C4+ materials.
There have been disclosed a limited number of methods directed to this area. U.S. Pat. No. 5,744,680 to Mulvaney, et. al., discusses recycling methane recovered from an OTO product as a diluent to reduce water in the reaction zone which was found to adversely affect the activity of the catalyst. U.S. Pat. No. 5,817,906 to Marker, et. al., talks about transforming an OTO feed alcohol to an ether and introducing the ether to the OTO reaction zone to reduce the amount of water contacted with a metal alumino-silicate catalyst to provide extended catalyst life. U.S. Pat. Nos. 5,914,433 and 6,303,839 to Marker assert that cracking C3+ olefins from an OTO reaction in a separate cracking zone over regenerated OTO aluminophosphate catalyst and passing the cracked product to the OTO reactor provides extended catalyst life in the oxygenate conversion zone.
U.S. Pat. No. 6,455,749 to Vaughn touches on the recycle of a heavy hydrocarbon fraction from an OTO reaction to an OTO reactor or separate reactor to convert at least a portion of the heavy hydrocarbons to light olefins. In that reference, it is disclosed that the conversion of C4+ olefins, in particular butene-1, is significantly lower than a typical oxygenate feedstock, in particular methanol, at the same reaction conditions over a silicoaluminophosphate catalyst:
In response to the aforementioned needs, and distinct from the noted references, the present invention simultaneously provides for increased life in the OTO reaction zone through reduced moisture content and improved utilization of C4+ olefins produced by the OTO reaction.